AirDrop not working after OS upgrades

Apple Support (thanks, Tom!) solved this for me. My Mac's Messages app was blocking my personal phone numbers, as spam. I certainly don't recall blocking them, and the issue came up only recently after upgrading the OS. But perhaps I had earlier, and it wasn't a problem until the upgrade.

Anyway, in my case the answer was to go into the Messages preferences, click the iMessage tab, then the Blocked tab, and then to delete my phone numbers from the list of spam numbers.

AirDrop is magical once more.

Well, whether or not the iPhone and the iMac report being connected, AirDrop isn't working. I.e. they can report being connected or not connected and it makes no difference. But thanks for your suggestion.

Anyone else have any ideas? The iPhones in our house can see each other but they no longer see the iMac, and it no longer sees them, since upgrading to High Sierra. Bluetooth and wifi are on, "Block all incoming connections" is off in Firewall, as recommended, and AirDrop is set on all devices to allow being discovered by Everyone.

Yes, it is unlocked. Indeed it has to be, as I (would) use it to transfer photos to my iMac.

One other thing is that although I try AirDrop only when the two devices report that they are connected by Bluetooth, the iPhone seems to have a hard time staying BT connected. That may be a clue.

I mention Bluetooth because Apple says it must be on: "Check that both of you [meaning the connecting devices] have Wi-Fi and Bluetooth turned on."

Apparently AirDrop uses Bluetooth to create a peer-to-peer Wi-Fi network between the two devices.